1. Field of the Invention
This invention generally relates to a microfilm reader apparatus for viewing microimages disposed on microfilm and the like and, in particular to a microfilm reader apparatus having a plurality of magnification lenses which cooperate with a single image rotation prism.
2. Description of the Prior Art
Many documents are microfilmed using various types of microfilm cameras, such as a planetary microfilm camera. Some documents which contain a voluminous amount of printed information may have to be photographed at a relatively high magnification level, resulting in the printed images being relatively small when projected. Thus, many microfilm reader apparatus are provided with two or more projection lenses to enable the projection of images at various magnification levels. When viewing microfilmed documents containing relatively small images, the operator merely switches to a relatively high magnification level lens to view the desired microimage.
Some reader printer apparatus are used primarily with microfilm or microfiche wherein the images are disposed parallel to the major axis of the frames. In such apparatus, the images are printed parallel to the long edge of the paper to avoid a portion of the image being cut-off. An example of such a machine is disclosed in copending U.S. patent application, Ser. No. 07/104,358 filed on Oct. 2, 1987, assigned to the same assignee as the present application. However, such apparatus may also be used with microfilm or microfiche wherein the images are disposed parallel to the minor axis of the frame. In this case, it is necessary to rotate the image ninety degrees such that the image is printed along the short edge of the paper.
Another problem resulting from the microfilming of documents is that the document may be skewed when photographed. This results in the microimage being skewed with respect to the edge of the microfilm. Accordingly, when the microfilm is inserted into a microfilm reader apparatus, a skewed image will appear on the viewing screen. Even though the image may still be read, it may be awkward for an operator. Moreover, sometimes the image is photographed upside down. To alleviate these problems, an image rotation prism is provided and disposed within an image projection path which cooperates with the projection lenses to provide an upright image at the desired magnification level on the viewing screen.
Since the prism will invert an image in the image projection path in the similar manner as a mirror, it is important that the prism be utilized with all of the projection lenses. In prior art apparatus, this was accomplished by providing a plurality of prisms and associating one prism with each of the projection lenses. Such an arrangement is disclosed in U.S. Pat. No. 4,494,840. In that patent, two projection lenses are provided with an image rotation prism positioned directly above each projection lens. The assembly of the projection lenses and prisms are movably mounted with respect to the image projection path such that one projection lens and associated prism can be moved into the image projection path at a time. Gears are provided such that rotation of either prism will cause identical rotation of the other prism such that the image is rotated by an identical amount regardless of which projection lens is disposed in the image projection path.
One of the problems associated with such a system as disclosed in U.S. Pat. No. 4,494,840, is that it requires the use of two image rotation prisms which are generally expensive and thus add to the overall cost of the device. Also, such a system is relatively more complicated since it requires a gearing arrangement between the two prisms to cause both prisms to be rotated by an identical amount.